DC - Rectangular/Oval Ducts?

[Rob Bodenschatz]

I've been thinking about dust collection for my new shop. I am planning to put in a wood raised floor over the concrete slab. I would like to run dust collection under this raised floor. If I used 2x6s on edge, I should be able to get a 5" pipe under it which would be sufficient for my shop. I'd like to explore the possibility of using 2x4s on edge to gain a few inches and cut down on cost a bit. If I do that, I obviously would not be able to use a 5" pipe. My question is, would it be proper/efficient to have rectangular ducts (maybe with rounded corners, or, even an oval shape) that carry the same amount of volume as a 5", but are shorter? I realize I may need to have these fabricated. The cost involved in doing that may kill the idea but I was wondering if you guys think that would work. Am I being clear here with my question?

[Jamie Buxton]

Air doesn't care whether it is flowing in a round duct or a rectangular duct.

For rectangular duct, you might consider standard HVAC duct which is intended to fit in 2x4 walls. Or you could build your own from plywood. Either is likely to be less expensive than getting a sheet-metal shop to custom-build stuff.

A third possibility is to use the floor "joists", concrete floor, and wood floor as your duct. They form a rectangular cross-section through which you could suck air. There's two cautions with this approach. First, you'd want to seal the duct so it doesn't have leaks. Caulk should do this. Second, if the cross-sectional area of your duct is too big, the air velocity slows, and the sawdust drops out of the airstream. The solution is to place your joists close enough together to approximate a 5" duct. If all this gets too complicated, go back to building separate plywood duct.

[Rob Bodenschatz]

You know Jamie, I thought of building the duct right in the floor like you suggest but wasn't sure if it would be a good idea. I guess it would depend on getting it sealed properly and having a smooth surface on the inside. That suggestion has been made on some other forums so I may give that some consideration. Seems like a pretty cost-effective way of doing it as well.

[Jim Becker]

Oval will work, but the transistions to/from round add resistance, etc. Rectangular will also work...same situation, but I'd also be concerned about the sharp corners "holding" material over time.

I'm one of those folks not in favor of duct work "in the floor" if it's not fully accessable. At least betwen the floor joists, you can provide a removable cover for the channel.

[Larry James]

Rob,

Round duct is the most efficient for air systems. Add dust to the air and I think you will have a much higher resistance system, and dust would be more likely to pile up along the edges and at turns and transitions.

A 10" by 3 1/4" standard "wall stack" (designed to fit a 2"x4" wall cavity) or a 4" by 6" duct is about equivalent to a 5" round pipe in HVAC duct design. However long sweep elbows and other fittings have much higher resistance to flow as duct shape moves away from round.

I would go with round, or oval as a distance second choice - just my opinion - based on teaching duct design for 20 years.

Larry

[Jeffrey Makiel]

Rob,
Larry is providing sound advice. I designed HVAC systems as an engineer and Larry's adivce is dead on. For air moving particulate matter (i.e., dust), use round or flat oval duct. Rectangular duct is not recommended.

Rectangular duct, like the typical 10 x 3-1/4 home heating duct, is meant for low velocity sytems. This width-to-height aspect ratio is almost violating a 3.5 to 1 aspect ratio . Since your dust collection system can easily run 5 times or more faster than a normal heating system, you will have a larger friction loss and a potential for dust to collect in the 'corners' of the duct.

Unfortunately, flat oval duct can be very expensive if your goal is to reduce the overall cost of your project. Round is always 'choice'.

-Jeff :)